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Area Forecast Discussion (AFD)

Issued by Eureka, CA (EKA)

FXUS66 KEKA 202253

National Weather Service Eureka CA
253 PM PST Sat Jan 20 2018

.SYNOPSIS...A stormy Sunday for NW California. Windy along the 
coast, rain elsewhere, and snow above 3000 feet. Winter Storm 
Warning for northern Trinity County, Winter Weather Advisory for 
southern Trinity County. Dry on Monday, with some rain chance on 
Tuesday. Next major system will be in store on Wednesday, then 
continued wet weather after that.


.DISCUSSION...Enjoy the calm before the storm. It was a nice day 
across NW California during the day on Saturday. With mostly sunny 
skies and few passing high clouds. One can see the snow cover from 
previous storms on the visible satellite imagery. Highs were in the 
low to mid 50s along the coast, 40s inland on Saturday. Do expect 
clear skies for this evening, then increasing clouds overnight. This 
will allow overnight temperatures to drop into the 20s and 30s 
inland tonight. 

Very active weather in store for Sunday as a strong weather front 
make landfall into NW California. Do expect winds to gusts of 35 to 
45 mph over the Coastal Plains, and gusts of 50 to 60 mph for the 
Mountains. The strongest winds will happen midday Sunday. A wind 
advisory is in effect for Crescent City and exposed coastal ridges.

Sunday will be a wet one also. Do expect rain total of around 1 to 2 
inches in the area, and higher amounts over windward facing slopes 
for the coast mountain. Do expect the heaviest rain to hit around 
the afternoon through the evening hours. Precipitation will taper 
off late Sunday night as the heaviest precipitation push south into 
San Francisco Bay Area and east into interior California. 

Now, let's talk about snow. Snow level will be around 2500 to 3000 
feet. It will be lowered during times of heavy precipitation. Do 
expect 4 to 7 inches of snow for 2500 to 3500 feet locations in 
Trinity County. Higher amounts will be found in locations above 3500 
feet. The heaviest snow will happen during the day on Sunday, and 
taper off during Sunday evening. The highest snowfall amounts will 
be over northern Trinity County, where there is an abundance of low 
level cold air. For southern Trinity County have a shallower low-
level cold air layer, and it will be eroded away quicker than 
locations further north. 

Looking at model sounding can provide some intriguing information. 
The sounding is saturated over the Dendritic Growth Zone (-10C to -
20C layer). However, the strongest rising motion (negative omega) 
does not coincide with the Dendritic Growth Zone. The strongest 
rising motion will be below the Dendritic Growth Zone. What this 
means is that the snow flakes may not have enough residence time to 
grow rapidly as the perfect snow flakes. Instead, we may see plates 
instead of Dendrites. If this is the case, then snow accumulation 
will be less than expected, as snow tend to be compressed if they 
are composed of mostly plates instead of dendrites. The snow physics 
will be a major factor in determining how much snow accumulation 
Trinity County will see. Right now, have a Winter Storm Warning for 
northern Trinity County, and a Winter Weather Advisory for southern 
Trinity County. Nonetheless, travel will be hazardous due to low 
visibilities from heavy snow, slippery road conditions due to wintry 

Confidence is high that it will snow in the interior. However, 
confidence is low to moderate on the snowfall amount. Please stay 
tuned for the latest forecast updates.

After the front clears the area, we will have a dry day on Monday 
across NW California. For Tuesday into Tuesday night, there are some 
chance of precipitation from Cape Mendocino north to Oregon border. 
The next major system will impact us on Wednesday, and then we will 
be dealing with lingering showers through the rest of the work week.


.AVIATION...VFR conditions will prevail at the forecast terminals
through tonight. There is a slight potential for IFR in fog at UKI 
early Sun morning due to moist grounds, light winds and semi-clear 
skies. Confidence is not high at the moment. Otherwise, VFR cloud 
layers are expected for the coastal terminals through tonight, 
though pockets of misty air may form this evening before SE winds 
aloft increase drying out the boundary layer. Conditions will 
deteriorate through the day on Sun at ACV and CEC as gusty S-SE 
winds ramp up and speed profiles become favorable for low level wind 
shear. VFR ceilings and vsbys should hold through early to mid 
morning Sun before conditions fall to MVFR or worse as the front and 
moderate rain moves onshore around mid day.


.MARINE...Seas have been slow to fall today. As of 2pm, all 
the buoys were running about 2 feet above the ENP guidance. Trend 
will be for this mid period NW swell subside tonight. Significant 
wave heights may not fall below 10 ft near shore til early Sun 
morning. There will not be a long window before seas start to ramp 
up again in response to robust southerly wind waves. The steep wind 
waves will combine forces with a building westerly swell to produce 
combined seas from 15 to 20 feet Sun afternoon and evening. Seas will
remain large and chaotic going into Sun night before significant 
wave heights start to abate through the day on Mon. An active 
pattern will likely maintain elevated and hazardous seas through 
most of next week.   


CA...Winter Weather Advisory from 8 AM to 10 PM PST Sunday for CAZ108.

     Wind Advisory from 10 AM to 4 PM PST Sunday for CAZ101-102-
     Winter Storm Warning from 8 AM to 10 PM PST Sunday for CAZ107.

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 3 AM PST Sunday for PZZ470.

     Gale Warning from 3 AM to 5 PM PST Sunday for PZZ470.
     Small Craft Advisory until noon PST Sunday for PZZ455.
     Gale Warning from noon to 6 PM PST Sunday for PZZ455.
     Small Craft Advisory until 9 AM PST Sunday for PZZ450-475.
     Gale Warning from 9 AM to 5 PM PST Sunday for PZZ450.
     Gale Warning from 9 AM to 6 PM PST Sunday for PZZ475.



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